When a substance changes from a solid to a liquid at its transition temperature it absorbs a quantity of heat known as the heat of fusion. This heat of fusion is released when the substance solidifies again. A substance having a large heat of fusion can be used as a heat sink. In the presence of temperatures in excess of the transition temperature of the fusible material the heat sink will remain at the transition temperature until all of the material has melted. Thus, a heat sink can offer protection against temporary overheating by storing the excess heat until it can be radiated away. It is sometimes used, for example, to protect electrical apparatus from temporary overheating. Each heat sink is designed to prevent a temperature rise above the safe temperature of the apparatus to be protected and is made large enough to store the expected quantity of excess heat. Since electrical apparatus often operates more efficiently at higher temperatures, the transition temperature of the fusible material should be below but as close to the safe temperature as possible.
Thus, a good fusible material for a heat sunk must melt at the desired temperature and have a large heat of fusion per unit weight or per unit volume, depending on whether weight or volume considerations are more important. Another important property of a suitable material is the absence of a tendency to supercool. One fusible material, sodium acetate trihydrate, for example, partially supercools each time it re-solidifies, leaving less and less material available for use as a heat sink. Supercooling can be avoided by seeding the melt, but the system is more complex.